Acoustic wave devices have demonstrated utility in a variety of sensing applications, including gas and vapor detection as well as sensing species in liquids. A two-port acoustic wave sensor includes an input transducer for generating an acoustic wave, an interaction region in which the propagating wave interacts with the environment, and an output transducer for detecting the wave. When an acoustic mode has significant amplitude at the surface, its propagation characteristics can be altered by changes in material on or near the device surface. The cumulative effects of such an interaction over the propagation path of the acoustic wave result in changes in wave amplitude and phase delay at the output transducer. In the simplest cases, acoustic wave devices function as highly sensitive detectors of changes in surface mass, responding primarily to accumulated mass per unit area. Specific sensors are achieved by securing a film capable of immobilizing a particular species from the environment to the interaction region of the device.
Reference is made to the following journal articles and U.S. patents as teaching various aspects of acoustic wave devices.
An article entitled "Characterization of SH Acoustic Plate Mode Liquid Sensors", by S. J. Martin, A. J. Ricco, T. M. Niemczyk, and G. C. Frye, SENSORS & ACTUATORS, Vol. 20, pp. 253-268 (1990) describes a two-port acoustic wave sensor that utilizes shear horizontal acoustic plate modes to probe a solid/liquid interface. The modes are excited and detected by interdigital transducers on thinned quartz plates.
In an article entitled "Monitoring Thin-Film Properties with Surface Acoustic Wave Devices: Diffusion, Surface Area, and Pore Size Distribution", by G. C. Frye, S. J. Martin, A. J. Ricco, and C. J. Brinker, CHEMICAL SENSORS AND MICROINSTRUMENTATION, American Chemical Society, Washington, D. C. 1989, pp. 208-221, there is described an ability of surface acoustic wave devices to monitor an absorption of N.sub.2 onto the surfaces of porous films and also the diffusion of species into polymer films.
In an article entitled "Mechanism of Operation and Design Considerations and Design Considerations for Surface Acoustic Wave Device Vapor Sensors", SENSORS & ACTUATORS, Vol. 5, pp. 307-325 (1984), H. Wohltjen describes characteristics of surface acoustic wave devices and techniques by which they may be employed as vapor sensors. The perturbation of surface acoustic wave velocity by polymeric coating films is also discussed. The article states that highest sensitivity can be achieved when the device is used as the resonating element in a delay line oscillator circuit.
In an article entitled "Surface Acoustic Wave Gas Sensor Based on Film Conductivity Changes", SENSORS & ACTUATORS, Vol. 8, pp. 319-333 (1985) by A. J. Ricco, S. J. Martin, and T. E. Zipperian there is described a surface acoustic wave sensor that functions via changes in the conductivity of a thin surface film.
U.S. Pat. No. 4,895,017, issued Jan. 23, 1990, entitled "Apparatus and Method for Early Detection and Identification of Dilute Chemical Vapors" to Pyke et al. describes a detector and method for identifying a chemical vapor and determining its concentration. The chemical detector includes a plurality of surface acoustic wave devices.
U.S. Pat. No. 4,726,225, issued Feb. 23, 1988 entitled "Surface Acoustic Wave Gas Flow Rate Sensor with Self-Heating Feature" to Brace et al. describes a surface acoustic wave device for measuring a mass flow rate of a gas. The surface acoustic wave device includes a surface acoustic wave delay line formed of a piezoelectric substrate.
In U.S. Pat. No. 4,598,224, issued Jul. 1, 1986 entitled "Surface Acoustic Wave Device for Sensing the Presence of Chemical Agents" Ballato describes a surface acoustic wave device for sensing the presence of chemical agents by chemo-electronic means.
U.S. Pat. No. 4,312,228, issued Jan. 26, 1982, entitled "Methods of Detection with Surface Acoustic Wave and Apparatus Therefor" to Wohltjen describes the monitoring of physical parameters relating to various fluids and polymers by contacting same with the surface of a piezoelectric material through which is passing a surface acoustic wave. An alteration of the wave is said to be an indication of the parameters.
U.S. Pat. No. 4,265,124 issued May 5, 1981, entitled "Remote Acoustic Wave Sensors" by Lim et al. describes an acoustic wave sensor that includes an acoustic wave oscillator having a resonant frequency that is modulated according to changes in a physical variable.
U.S. Pat. No. 4,096,748, issued Jun. 27, 1978 entitled "Surface Acoustic Wave Strain Detector and Gage" to Sallee describes a strain sensor that includes an oscillator having a surface acoustic wave delay line as a frequency controlled element.
The following patents are cited as being of general interest in the area of sensor technology.
U.S. Pat. No. 4,818,348, issued Apr. 4, 1989, entitled "Method and Apparatus for Identifying and Quantifying Simple and Complex Chemicals" to Stetter discloses the use of a computer controlled array of chemical sensors. U.S. Pat. No. 4,295,102, issued Oct. 13, 1981, entitled "Surface Acoustic Wave Sensor Sensing Circuits" to Schmidt et al. describes a surface acoustic wave sensor system in which a predetermined level of coupling between two surface acoustic wave arrays is intentionally provided so as to couple RF energy from one into the other. U.S. Pat. No. 4,759,210, issued Jul. 26, 1988 entitled "Apparatus for Gas-Monitoring and Method of Conducting Same" by Wohltjen et al. describes a method of monitoring a gas that includes trapping means, such as a tube or other conduit, through which the gas to be monitored is passed. A sorbent mass is provided to intercept the gas passing through the conduit. Chemical sensors are provided in an array for monitoring the gas after it emerges from a conduit. U.S. Pat. No. 3,983,424, issued Sep. 28, 1976, entitled "Radiation Detector Employing Acoustic Surface Waves" to Parks describes a radiation detector having an acoustic transmission line with transducers on a substrate for providing an acoustic surface wave in the substrate. A phase change in the wave is said to be a measure of the temperature change, resulting from absorbed radiation, at the surface of the substrate. Finally, U.S. Pat. No. 2,912,854, issued Nov. 17, 1959, entitled "Ultrasonic Surface Testing Device" to Schubring describes a surface testing device that employs ultrasonic energy.
What is not taught by this prior art, and what is thus an object of the invention to provide, is a multiple-frequency surface acoustic wave device for chemical sensing and materials characterization in both gas and liquid phase.
It is a further object of the invention to provide method and apparatus to detect the presence and/or concentration of a chemical species or substance in the gas or liquid phase, or to characterize one or more physical properties of a thin film of material, by employing an acoustic wave (AW) sensor that operates simultaneously or sequentially at a plurality of distinct frequencies.
It is one further object of the invention to provide a multiple-frequency AW sensor operable for differentiating a response due to one physical perturbation from the response due to another physical perturbation.